1. Field of the Invention.
The present invention relates to improvements in a method for evaluating residual fatigue life of mechanical parts.
2. Description of the Prior Art.
Heretofore, in a process of maintenance and control for mechanical parts a crack finding inspection was carried out, but according to this inspection it was only possible to determine whether the parts have reached their useful life or not. As methods for estimating approximately when cracks will be generated from a result of an inspection conducted prior to generation of cracks, there can be conceived a method of detecting changes in the nature of material at the surface of mechanical parts prior to generation of cracks and a method of observing micro cracks, but either of these methods is poor in precision, and there has been almost no example of success as a practical method.
As a representative one of the methods for detecting changes in the nature of material, there is a method of making use of the X-ray diffraction process and paying attention to changes in a profile of the X-rays diffracted from a part. A description will now be made outlining this method with reference to FIG. 7.
FIG. 7 is a schematic diagram showing a general X-ray diffraction intensity curve, and a half-width indicated in this figure means a width of a peak at a 1/2 height of the peak in a profile of the diffracted X-rays. Representing the half-width by H and that before use of the part by Ho, then as shown, for example, in FIGS. 8(a) and (b), a halfwidth ratio H/Ho at the surface of a test sample has a good correlation to a fatigue damage ratio N/Nf (N: number of repetitions of stress, Nf: number of repetitions of stress at fracture). Accordingly, a degree of fraction of fatigue life can be estimated from a half-width ratio.
However, as it is seen in FIGS. 8(a) and (b), although change of a half-width H/Ho occurs remarkably in the first period I, the gradient of change of H/Ho with respect to a fraction of fatigue life N/Nf in the second period II which occupies most of the fatigue life is small. Therefore, precision in prediction of the fatigue life is poor. Also, in general cases, the initial value of Ho is not knwon for the mechanical parts except if Ho was measured before service. Therefore, Ho should be estimated by any method, though the reliability of predicted fatigue life depends strongly on the accuracy of the estimated Ho.
These two reasons have made the method utilizing X-ray diffraction measurement at the surface of mechanical parts unsuccessful. The third period III in FIGS. 8(a) and (b) involves the process of generation and propagation of macroscopic cracks, and this period is outside of the object of inspection according to the present invention.